Photo sensors are used to control switching on and off by sensing light or to control the illuminance of backlights of cellular phones and the like. Further, photo sensors are used as sensors for converting video pictures or graphics into electrical signals in a fax machine, a photocopier, a video camera, a digital still camera, and the like.
Photo sensors convert light into electrical signals in such a way that photo charges generated by irradiating a photoelectric conversion layer with light are outputted as an electric current in accordance with the amount of irradiation light. When slight difference of the photo charges between the respective photoelectric conversion layers is difficult to be expressed as signals, it is necessary to amplify the outputted signals with a circuit for amplifying the photo charges (hereinafter referred to as an amplifier circuit) so that the slight difference can be clarified.
In the case of manufacturing a photo sensor equipped with the amplifier circuit, a thin film transistor 1102 is first formed as an element constituting a part of the amplifier circuit over a substrate 1101 as shown in FIGS. 11A and 11B. Next, a cathode electrode 1103 of a photoelectric conversion element 1106 is stacked so as to connect with a source electrode or a drain electrode of the thin film transistor. After that, a silicon layer 1104 and an anode electrode 1105 are formed over the cathode electrode 1103, and thus the photo sensor is manufactured. Such a method is common to form the photo sensor (for example, Reference 1: Japanese Patent Document Laid-Open No. 2001-265283, FIG. 13).